MILFORD, Conn. (AP) â€” A 17-year-old Connecticut boy charged with stabbing a classmate to death in school on their prom day pleaded not guilty to murder Wednesday as his attorney said he was investigating a possible mental health defense.

Christopher Plaskon, who answered a judge’s questions in a firm voice, entered the plea in Milford Superior Court. He also waived his right to a probable cause hearing and elected for a trial before a three-judge panel rather than a jury.

Plaskon’s attorney, Richard Meehan, said after the hearing that it would be premature to say he’s going to mount a mental health defense, but he said doing so would be more difficult before a jury. Plaskon, whose family attended the hearing, is on anti-psychotic and anti-anxiety medications but is lucid, Meehan said in court.

“Obviously we have an obligation to this young man based on the things that we do know to evaluate what his mental status was,” Meehan said. “I’m a firm believer juries have a difficult time with issues pertaining to mental health in the criminal justice system.”

While jury verdicts must be unanimous, a verdict by judges can by 2-1, Meehan said. A trial is not expected until at least the fall, he said.

Prosecutor Kevin Lawlor said his job was to prove the case and that a possible mental health defense was irrelevant at this point.

Plaskon is charged with fatally stabbing his friend Maren Sanchez in a hallway at Jonathan Law High School in Milford in April. The 16-year-old was a member of the National Honor Society who was active in drama and other school activities.

The attack occurred hours before the school’s junior prom. Police are investigating whether it was related to Sanchez’s refusal to be Plaskon’s prom date.

Plaskon’s family has said it mourned the loss of life and was struggling to understand what led to the crime.

“This is a very sorrowful, sad youngster,” Meehan said. “It really is a double tragedy.”

A witness saw Plaskon on top of Sanchez during the attack and tried unsuccessfully to pull him away, and another saw him discard a bloody knife, according to a police affidavit.

Plaskon, his clothes bloody, was taken to the principal’s office, where an officer arrived and asked what happened. “I did it. Just arrest me,” Plaskon replied, according to the officer.

Plaskon, who turned 17 last week, is being held on $3 million bond at Manson Youth Institution in Cheshire, where he was placed at a hospital.

He is being prosecuted as an adult and faces up to 60 years in prison with a mandatory minimum of 25 years if found guilty of murder.